The present invention relates to conditioning a carbon rich material, in particular coal, for the purpose of obtaining a product which can be easily transported through pipes and which is easy to store over long periods of time. More particularly, it relates to a process for conditioning a carbon rich material applicable, preferably but not exclusively, to the injection of auxiliary fuel into the nozzles of a blast furnace of the type used in the iron industry.
Processes for conditioning a carbon rich material of the above mentioned general type are known in the art. The injection of auxiliary fuel into the nozzles of a blast furnace is a practice which has been known for a long time and utilizes a hydrocarbon, such as fuel oil. Research and development effort is presently being exerted with a view to replacing, at least partially, fuel oil with finely ground coal.
In the present state of the art, two techniques will appear to be accepted by industry within a few years.
The first one consists of blowing dry coal particles from a storage hopper to the nozzles by means of a carrier gas. This technique which makes special use of the pneumatic conveyance of powders in a dense phase and can therefore be considered as one that can be or has already been mastered. It has however, several drawbacks which, although they are not restrictive, may nevertheless slow down the development of the process.
The main drawback involves the conditioning of the coal itself into fine particles. Very careful precautions must indeed be taken in order to avoid risks of explosion which might result from the suspension of particles into the atmosphere all through the process which starts with grinding followed by storage and then transportation. Furthermore, at the grinding stage itself, which is carried out in the dry state, it is necessary to dry the initial product which expends energy.
The second technique consists of conditioning the finely ground coal by suspending it in a liquid phase (generally water), in order to form coal-water mixture having a high coal concentration usually known as "pulp". The idea seems very attractive on the surface for a pulp of this type is usually a product which has characteristics, regarding transportation, that are analogous to those of a liquid and thus making it possible, in principle, to use lines that may already be in existence without any major modifications for the transportation of fuel oil.
The idea also seems attractive because of hydrogen enrichment of the gas recovered at the top of the apparatus through dissociation of the injected water. Nevertheless, this aspect of increasing the value of the top gas can only be of secondary importance for economic considerations show that the process is of interest only for high values of the coal concentration in the pulp such as 70% and higher (by weight). Now, studies carried out to this day seem to indicate that possibilities at that level become rapidly limited due to the very fast increase in viscosity of the pulp at values greater than 75% by weight of the coal, which then makes the pumping and circulation thereof difficult in pipes using the means usually available for those purposes.
Another drawback is concerned with storage, which may result from the tendency of the coal powder to settle out over more or less prolonged periods of time.